1. Field of the Invention
The present invention relates to a photographic camera having an automatically focused adjustable focus lens in general, and to an automatically focused motion picture camera, in particular.
2. Description of the Prior Art
Control systems for automatically positioning the movable element of an adjustable focus lens that is adjustable over a given operational range to a desired focus position within said operational range in order to properly focus an image of a remote object at the image plane of a photographic camera in response to a signal representative of the distance to said remote object, have been disclosed in the prior art. A camera having such a focus control system that utilizes acoustical energy to derive a signal representative of the distance to said remote object is described in my copending application Ser. No. 965,404, filed Dec. 1, 1978.
In an adjustable focus lens autofocusing motion picture camera having a focus control system of the type mentioned above that is adjustable over a given operational range, the focus control system positions the movable element of said lens to the proper focus position from a previous focus position within said range in accordance with a rangefinder derived subject distance signal. If the desired focus position is relatively close to the preceeding focus position, the amount of time required to position the movable element of said adjustable focus lens to the desired focus position is relatively small. However, once the focusing distance between the preceeding focus position and the desired focus position exceeds a predetermined magnitude, the amount of focusing time required becomes excessive in that a subject remains out of focus for a length of time that tends to be objectionable to, for example, one who subsequently views a motion picture that has been produced with this type of adjustable focus lens focusing system.
One approach to reducing the time required for an adjustable focus lens to be automatically focused on a subject to be photographed would be to increase the focusing speed of the drive means that focuses said lens. A reduction in focusing time would be achieved by such an approach. However, subjects that require relatively small amounts of lens focusing movement would come into focus too rapidly to provide the desired smooth transition between an out-of-focus and an in-focus subject image.